In bacteria and yeast, many of the genes important in the cellular response to DNA damage are induced by such damage. Until recently, evidence for such specific responses to genotoxic stress in mammalian cells was not definitive. In the last year, our group has been in the forefront in demonstrating that certain genes in mammalian cells can be specifically induced by specific types of DNA damage and that SOS-like responses exist in mammalian cells. We developed a hybridization subtraction technique to isolate cDNA clones coding for DNA-damage-inducible (DDI) transcripts, and have isolated cDNA clones for more than 20 different new DDI genes. Most were only induced by DNA damage and not other types of stress such as heat shock. We have sequenced most of these cDNA clones and recently have isolated genomic clones for some of these sequences. One cDNA clone, DDIA18 was found to encode an mRNA only induced by DNA-damaging agents repaired by nucleotide excision (UV-type) repair, and was found to code for a single-stranded-DNA binding protein. cDNA clones for both human and rodent DDIA18 mRNA have been sequenced; the predicted peptide sequence has been very highly conserved which implies that this protein plays an important cellular role. Several other of our DDI genes were found to be coordinately induced by either DNA damage or inhibition of cell growth. There is good evidence in both bacteria and eukaryotes that inhibition of cell growth after DNA damage can have a protective effect; e.g., one of the SOS genes is a growth arrest gene. In collaboration with D. Nebert, we have found that these genes were coordinately overexpressed in a mouse mutant which may provide insight into their regulation. Two of these genes have been sequenced and newly described regulatory regions have been tentatively identified; antibodies to one of the proteins have been developed. In collaboration with I. Hickson and L. Barrows, expression of our DDI transcripts in DNA repair mutants has been investigated. Several examples of both increased and decreased expression transcription in the mutant cells has been observed. The functions of most of our DDI sequences are unknown, but it is probable that the protein products of at least some of these transcripts play a role in the cellular response to DNA damage.